Of late years transparent dentrifice formulations of various types have been developed and put on the market, giving a favorable impression with their freshness and cleanliness associated with the transparency.
For transparent dentifrice formulations there has been used in the past a silica base material which can impart transparency to the dentifrice formulation, but has no substantial abrasiveness so that the resulting dentifrice has not been satisfactory in actual use as a dentifrice. Further, it has also been proposed to use a silica base material which has a refractive index which is close to that of the transparent dentifrice vehicle used for formulation into a transparent dentifrice, but the known silica base material has a refractive index that shows fluctuation and is poor in stability so that it is difficult to obtain a transparent dentifrice with good stability.
Meanwhile, several methods have been proposed to produce silica base materials having appropriate abrasiveness for transparent formulation. For example, Japanese Patent Publication No. '74-11159 describes a method wherin a commercial superfine amorphous silica devoid of abrasiveness and therefore unsuitable for dentifrice base material is wetted with water or a dilute aqueous solution of inorganic alkali metal salt, fired at 500.degree.-1000.degree. C., and then ground. It is true that this method provides abrasiveness needed by a dentifrice base material. But the abrasiveness thus provided is often so much in excess as to damage tooth enamel and the material itself does not give good transparency and consistent stability. Finally, it would be hard to industrialize the method economically.
Another method is described in Japanese Patent Kokai Nos. '76-12869 and '76-136841 to provide abrasiveness to silica. The material obtained by this method, however, does not give good paste stability when combined with a transparent dentifrice vehicle.
The invention described in Japanese Patent Publication No. '73-14935 discloses a transparent dentifrice formulation containing silica specified as follows:
refractive index: 1.40-1.47
oil absorption: 1.5 cc and less
particle size: ca. 0.01-0.5.mu.
The silica prepared by the method disclosed in this publication gives a BET surface area of 150 m.sup.2 /g, and a CTAB surface area of 82 m.sup.2 /g. When this silica is mixed with humectant and left to stand, the turbidity of the mixture, or paste, grows markedly from day to day and the paste stability is gradually worsened.
In addition, one of the present inventors disclosed in Japanese Patent Publication Nos. '74-8640 and '77-15078 a method to control the refractive index of silica by allowing fluoride ion to intervene in the proposed process. The product of this invention as a base material for transparent dentifrice formulation was still not without a problem in transparency and was not as stable as expected.
Thus the silicas publicly known heretofore have defects either in abrasiveness, stability or transparency which are the essential characteristics of the base material for transparent dentifrice formulation, and therefore none of them are satisfactory for such formulation.
Years of intensive study on these essential characteristics of silica base material has now revealed to the present inventors that combinations of a tooth-paste vehicle with the silica base material of the present invention, as described in detail below, can bring forth a transparent dentifrice formulation with proper abrasiveness, water-like transparency and good long-term stability under storage.
Transparency as referred to in this invention is defined and measured as follows: two solutions or different refractive indices like glycerine and water are mixed in various proportions to give dispersion media of different refractive indices; a fixed amount of silica base material is then mixed with a fixed amount of each medium into a dispersion, which, as needed, is deaerated and subjected to measurement of refractive index and turbidity; and with these as parameters a curve is plotted to determine the minimum turbidity point which represents the transparency here.
The present inventors studied in detail the factors influencing the transparency and, finding that the porosity of silica base material is greatly involved, came to accomplish the invention.